We studied the influence of active force development on the lattice formed by the insertion of the thin filaments into the network of protein comprising the Z-line which marks the boundary between adjacent sarcomeres in striated muscle (the Z-I lattice). X-ray reflections from the A-band myofilament lattice and the Z-I lattice were recorded simultaneously as a function of the longitudinal force generated by the muscle. Our data indicate that changes in Z-I lattice spacing roughly follow those in the A-band lattice spacing as a function of sarcomere length and with activation. Another finding is that there is an activity dependent swelling in lattice spacing that could explain many of the discrepancies in the literature since up to now there have been no attempts to control for such behavior. The high flux multilayer beam at SSRL allowed us to collect fiber diffraction data in subsecond intervals allowing us to characterize this swelling as a function of contraction time.